The present invention relates to a method of making a stable anaerobic//UV/visible light curable adhesive that involves the step of the addition of a photoinitiator without a chelator.
Anaerobic cure compositions with secondary curing mechanisms, such as ultraviolet/visible light, are well known in the prior art. Such adhesives have a variety of commercial uses. Generally, these adhesives are especially useful for bonding tight-fitting metal parts, ideally 5 mil or less in size, wherein some amount of adhesive will be forced out of the bondline. In such applications, the ultraviolet/visible light is used to cure the portion of the adhesive that is forced out of the bondline. One use of an anaerobic/ultraviolet dual cure adhesive is in the manufacture of ammunition. In particular, such dual cure adhesives are useful in bonding the primer cup in its pocket in the cartridge of center fire ammunition. This bonding is critical in that an improper bonding of the primer cup can result in the introduction of moisture and other foreign materials into the primer cup pocket and cause a misfire of the ammunition. The dual cure nature of the adhesive provides anaerobic curing on the internal portions of the primer cup and ultraviolet/visible light curing on the exposed edges of the primer cup. The ultraviolet/visible light curing also provides the benefit of sealing the area around the perimeter of the primer cup so that moisture can not be introduced into the primer cup pocket. U.S. Pat. No. 6,460,464 further describes the dual cure sealing of ammunition and is incorporated herein in its entirety. Other uses for such a dual cure adhesive include permanently fixing the position of a screw that is set at a particular point. In such applications the adhesive is placed along the threads and over the head of the screw. The anaerobic curing bonds the screw threads in place and the ultraviolet light is utilized to cure the adhesive over the head of the screw to prevent tampering.
Processes for manufacturing single and dual cure adhesives are also well known in the prior art. U.S. Pat. No. 4,235,986 describes a method of manufacturing a stable anaerobic curing adhesive and is incorporated herein in its entirety. A process for manufacturing a dual cure adhesive, wherein one of the curing mechanisms is anaerobic, is needed because of the difficulty in maintaining stability and performance in the anaerobic portion of the adhesive over time.
The reason for this is that anaerobic adhesives are extremely sensitive to the presence of trace transition metals that are found in virtually all materials. The manufacture of anaerobic adhesives thus requires particular procedures (See U.S. Pat. No. 4,235,986) and ingredients such as selected chelating agents, to prevent trace transition metals present in the manufactured adhesive from causing cure to take place in the absence of air before use. Thus, when an adhesive is manufactured it is stable until the time of application when it is brought into contact with metal substrates that will begin the curing process. Therefore it is critical that an anaerobic adhesive not come into contact with any source of transition metals after manufacture and before use. If one wishes to include an additive in the composition, it must be subjected to the manufacturing process in order for the adhesive to have acceptable stability. However, some additives such as photoinitiators are too sensitive to the conditions of the anaerobic manufacturing process and cannot be combined with the other ingredients. These manufacturing conditions may be oxidative and may include exposure to heat or light. If these materials are added to the anaerobic composition after the completion of the anaerobic manufacturing process, the trace metal contamination found in them may render the final product unstable or diminished in performance over time.
Therefore it would be advantageous to provide a manufacturing process for a stable anaerobic//ultraviolet/visible light dual cure adhesive with undiminished anaerobic performance that would allow for the addition of a significant quantity of one or more photoinitiators after the anaerobic manufacturing step is completed.
The present invention is directed to a process for manufacturing a dual cure anaerobic//ultraviolet/visible light adhesive. The process includes the first step of mixing polymerizable (meth)acrylate ester monomers or oligomeric polymerizable (meth)acrylate esters or both, (meth)acrylic acid, a chelating agent and saccharin and heating them at a specified temperature for a fixed time. The second step involves adding a tertiary amine accelerator, continuing heating of the mixture at the same temperature while rapidly stirring in air over an extended time to provide a stable yet reactive anaerobic composition. Next, after cooling to room temperature, one or more photoinitiators are added to the composition together with an oxidant. Unexpectedly, the final composition is stable, reactive anaerobically over 12 months at room temperature and capable of being cured by ultraviolet/visible light.